Cells of the immune system, most prominently B lymphocytes and macrophages, respond vigorously to a number of bacterial components, including lipopolysaccharide (LPS), and peptidoglycan. Presumably, this is an important element in the ability of vertebrate organisms to defend against bacterial infections. Experiments are proposed to identify the cellular receptors for the bacterial activators and to analyze the biochemical mechanisms used by these cellular receptors to signal to the cell that it has encountered them. Experiments are described to test whether serum components such as antibodies and complement are required for cellular responses to bacterial activators or whether cells have receptors for the bacterial activators themselves. Monoclonal antibodies recognizing some of the receptors will be obtained by screening for antibodies that can either block or mimic the response to bacterial activators such as LPS. Antibodies recognizing a particular receptor will be determined either by direct competitive binding studies or by genetic studies using the antibodies to select for receptor negative mutants of a responsive cell line. Previous results showed that in one B cell line and in one macrophage cell line the LPS receptor activates the Gi signaling component that inhibits adenylate cyclase. The generality of this mechanism will be examined both with other types of B cells and with other bacterial activators. The relevance of Gi signaling for the biological responses will be tested by inhibiting Gi by Pertussis toxin treatment of intact cells. Possible alternative signaling mechanisms involving protein kinase C will be examined. Finally, experiments are described to test whether inhibition of adenylate cyclase by Gi is the relevant pathway or whether Gi might be influencing cellular physiology by an alternative mechanism.